The present invention relates to a moving-coil electromagnetic actuator and, in particular, to an actuator for a valve for controlling the injection of fuel or fuel oil.
In the field of fuel-injection control valves, there are known actuators of the electromagnetic type which comprise a fixed electrical winding (coil) fixed firmly to the valve body. In such an actuator, a movable armature of ferromagnetic material having one end connected to a closure member of the valve is arranged coaxially with the winding and can slide (inside the winding) under the effect of the electromagnetic field generated by the winding when an electric current flows through it, bringing about opening and closure of the valve. A biasing spring is provided for bringing the armature to a rest position in the absence of electromagnetic operation, for example, to reach a valve-closure position.
The main problem with known devices is that they cannot be operated very rapidly because of the high inertia of the components.
The energy required to bring about the movement of the armature, and hence the travel of the closure member connected thereto, is directly proportional to the masses of the moving components and to the desired speed of execution of the operation. The mass of the movable armature of ferromagnetic material cannot be reduced beyond a particular limit because it is responsible for the force produced, and the mass of the biasing spring also partially determines the inertia which the electromagnetic operation has to overcome.
In order to generate the magnetic field necessary to bring about a rapid movement of the armature within a short time, it is therefore necessary to force a current of high intensity into the winding, to overcome the overall inertia of the moving parts, the pressure of the spring, and possibly that of the fuel or fuel oil; this requires a correspondingly high voltage, which is normally greater than the battery voltage available in motor vehicles.
The fixed valve core and the movable armature, both of which are made of ferromagnetic material, are thus subject to strong parasitic currents generated by magnetic induction and therefore (at least for the fixed core) have to be made of sintered material to limit this effect as far as possible, further increasing the costs and size of the device.
In these conditions, the inductance of the coil is normally high and the reactive component absorbs and stores a further quantity of energy proportional to the square of the intensity of the current flowing through it.
The rapid actuation times of the device which can be achieved by optimizing all of the parameters do not, however, permit multiple precise injections in close succession.
There may be further disadvantages owing to the range of temperature variation to which the device is subject in operation, which is due both to the large currents passing through it, and to the temperature of the engine environment.
Also known in the art are moving-coil electromagnetic actuator devices of the type comprising a magnetic core fixed to the body of the device and an electrical winding (a coil) immersed in the magnetic field produced by the core and movable relative to the core.
When an electric current flows through the winding, the winding translates rigidly, at a speed proportional to the magnetic induction, to the length of the wire constituting the winding, and to the current intensity. It is connected mechanically to a member to be actuated, so as to transfer thereto every stress (travel) to which it is subjected. A resilient reaction element is connected to the winding and to the member actuated thereby and is arranged to bring both of them to a rest position in the absence of an activation control.
As in the previous case, the mass of the resilient reaction element affects the efficiency of the device in terms of speed and energy, limiting its response rate upon activation. A fixing system is also required and this further complicates the device and makes it heavier.
A further aspect which affects the complexity of the device and its cost relates to the electrical connections which connect the winding to a fixed electrical driver circuit, and which have to be movable relative to the driver circuit in order to follow the travel of the winding.
The aim of the present invention is to provide a satisfactory solution to the problems set out above, overcoming the disadvantages of the prior art.
According to the present invention, this aim is achieved by means of an actuator device, particularly for a control valve, having the characteristics recited in claim 1.
In summary, the present invention is based on the principle of forming the resilient reaction element, in a moving-coil electromagnetic actuator, by means of the electrical winding itself, by taking advantage, in particular, of the helical configuration which is common to both and thus reducing the weight of the movable portion of the device so as to permit a fast response rate of the system, even with low operating currents.
The resilient element and the helical moving coil which are combined in a single member hereinafter defined as a whole as the actuating member of the actuator device, have a first, fixed end portion, fixed firmly to the body of the device and a second end portion which is movable away from or towards the fixed portion and is mechanically connected to the member to be controlled (for example, the closure member of a control valve).
According to the currently-preferred embodiment, the actuating member is formed in a two-layered helical configuration (that is, as a double winding), both ends of which are disposed in the region of the fixed portion of the actuating member thus formed, and are connected to respective electrical connection terminals that are also fixed.
An outwardly-extending helical section constituting a first layer extending from a first connection terminal as far as the movable end portion, and a return helical section constituting a second layer, arranged coaxially in series with the previous section, preferably wound outside it, and extending, still with the same direction of winding, from the movable end portion to the second connection terminal, are defined relative to the above-mentioned terminals.
The electrical winding is immersed in a strong fixed magnetic field generated by a permanent magnet.
Since the electrical winding also has to perform the function of a resilient element, it is no longer subjected to a rigid translational movement, but to an extension and contraction movement, in which the fixed end portion constitutes the reference relative to which this movement is performed.
The solution described thus advantageously solves the problem of the prior art devices since, as indicated, the configuration adopted enables both of the electrical connection terminals to be extracted in the region of the same end portion of the actuating member and also enables the terminals to be fixed